Disc drive memory systems are used by computers and currently also widely used by other devices including digital cameras, digital video recorders (DVR), laser printers, photo copiers and personal music players. Disc drive memory systems store digital information that can be recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Spindle motors had in the past used conventional ball bearings between the hub and the shaft and a thrustplate. However, the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc. Currently, the close proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, damaging the disc drive and resulting in loss of data. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings on the shaft and the thrustplate to support a hub and the disc for rotation.
In a fluid dynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Dynamic pressure-generating grooves (i.e., hydrodynamic grooves) formed on a surface of the fixed member or the rotating member generate a localized area of high pressure or a dynamic cushion and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing and between the rotating surfaces, enabling the spindle to rotate with a high degree of accuracy. Typical lubricants include oil and ferromagnetic fluids.
The shape of the hydrodynamic grooves is dependant on the pressure uniformity desired. The quality of the fluid displacement and therefore the pressure uniformity is generally dependant upon the groove depth and dimensional uniformity. As an example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure.
One known method for producing dynamic pressure-generating grooves presses and rolls a ball over the surface of a work piece. A problem with this method is the displacement of material in the work piece, resulting in ridges or spikes along the edges of the grooves. Removing these ridges is time consuming costly. A further problem is that the demand for higher disk drive rotational speeds requires the shaft and hub work pieces to be made of material that is as hard or harder than the material of the ball.
Another known method for producing the grooves of a fluid dynamic bearing uses a metal-removing tool and a fixture that moves the workpiece incrementally in the direction in which a pattern of grooves is to be formed. This approach also is not typically suitable for use with harder metals. Moreover, because each groove or portion of a groove must be individually formed and the workpiece then moved, the process is time consuming. Further, the equipment necessary for this approach is expensive and the metal-removing tool is subject to wear and requires frequent replacement.
Another known method for producing grooves involves an etching process in which the workpiece is covered with a patterned etch resistant coating prior to etching so that only the exposed portions of the workpiece are etched. One problem is the time consumed in applying and patterning the etch resistant coat. The resist coat must be baked to prior to patterning or etching. Another problem is that the coating must be removed after etching. This is frequently a difficult task, and one that can leave resist material on the workpiece surface resulting in the failure of the bearing and destruction of the disc drive. Yet another problem is that the process requires the extensive use of environmentally hazardous and toxic chemicals including photo resists, developers, solvents and strong acids.
Accordingly, there is a need for a method for forming accurate grooves in a work piece that does not require the use of a metal-removing tool that must be frequently replaced and does not use etch resistant material that could contaminate the work piece. As the result of the above-mentioned groove forming concerns, electrochemical machining (ECM) of grooves in a fluid dynamic bearing has been developed. The ECM process is generally known. However, the ECM process raises the need to accurately and simultaneously place grooves on a surface across a gap which must be very accurately measured, as the setting of the gap will determine the rate and volume at which metal ions are carried away from the surface. Deficiencies in mechanical tolerances may cause misalignment of the electrode with the work piece, causing an uneven gap and correspondingly uneven depth hydrodynamic groove. It is extremely difficult to make a tool with fixed electrodes that will guarantee a consistent work piece to electrode gap to form dimensionally consistent hydrodynamic grooves. Known methods to adjust electrodes (axially) include a worm and gear arrangement, which generates significant friction and is not reliably accurate. Some groove forming methods require the use of a coordinate measuring machine (CMM) to change the electrode. The centerline of the electrode has to be determined, and the work holder is positioned to match the centerline of the electrode, which has proven to be unreliable. Therefore, a need exists to reliably and repeatedly be able to set an accurate gap between an electrode and an interior surface of a work piece, in order to establish accurate grooves on the work piece.